close

Вход

Забыли?

вход по аккаунту

?

JP2018026445

код для вставкиСкачать
Patent Translate
Powered by EPO and Google
Notice
This translation is machine-generated. It cannot be guaranteed that it is intelligible, accurate,
complete, reliable or fit for specific purposes. Critical decisions, such as commercially relevant or
financial decisions, should not be based on machine-translation output.
DESCRIPTION JP2018026445
Abstract: PROBLEM TO BE SOLVED: To provide a piezoelectric element in which the influence of
residual stress of a piezoelectric thin film is suppressed, the restriction of the outer shape is
eliminated, and the signal noise ratio is improved with high sensitivity. A piezoelectric thin film
(3a, 3b) has a laminated structure, and a plurality of piezoelectric elements in which electrodes
(4a1, 4a2, 4b1, 4b2, 4c1, 4c2) are arranged on each piezoelectric thin film are formed. The
piezoelectric elements are disposed so as to overlap in the vertical symmetry. According to this
structure, the residual stress and the piezoelectric voltage generated due to the temperature
fluctuation are mutually offset by the output of the piezoelectric element overlapping in the
vertical symmetry, and the signal noise ratio is improved. [Selected figure] Figure 2
Piezoelectric element
[0001]
The present invention relates to a piezoelectric element, and more particularly to a high
sensitivity and low noise piezoelectric element.
[0002]
In recent years, microphones using MEMS (Micro Electro Mechanical System) technology which
is small in size and thin and have high-temperature processing resistance in a solder reflow
process for assembly are widely used for smartphones whose demand is rapidly expanding.
04-05-2019
1
In addition to MEMS microphones, other MEMS devices are rapidly spreading in various fields.
[0003]
Most of this type of MEMS element is a capacitive element that captures the vibration
displacement of the diaphragm due to acoustic pressure or the like as a capacitance change with
the opposing fixed plate, converts it into an electrical signal, and outputs it. However, in
capacitive elements, improvement in signal-to-noise ratio is becoming limited due to acoustic
resistance caused by air flow in the gap between the diaphragm and the fixed plate.
[0004]
Therefore, attention has been focused on a piezoelectric element that can extract acoustic
pressure or the like as a voltage change due to the distortion of a single diaphragm made of a
piezoelectric thin film.
[0005]
By the way, in the piezoelectric element, it is known that the residual stress or temperature
fluctuation of the piezoelectric thin film is output as an unnecessary signal to deteriorate the
characteristics when there is no acoustic pressure or the like.
Then, the technique of releasing a residual stress is employ | adopted by employ | adopting the
cantilever structure which makes one end of a piezoelectric thin film a free end (for example,
patent document 1).
[0006]
FIG. 9 shows a cross-sectional view of a piezoelectric element having a cantilever structure. As
shown in FIG. 9, piezoelectric thin films 3a and 3b of a multilayer structure are fixed to a silicon
substrate 1 as a support substrate through an insulating film 2, and the piezoelectric thin film 3a
is formed of electrodes 4a and 4b from above and below. Has a structure in which it is
sandwiched by the electrode 4b and the electrode 4c. Each of the piezoelectric thin film and the
electrode has a rectangular planar shape, one end is fixed to the silicon substrate 1 and the other
04-05-2019
2
end is a free end. The electrode 4a and the electrode 4c are connected to one wiring electrode 5a,
and the electrode 4b is connected to another wiring metal 5b.
[0007]
In such a piezoelectric element, when the piezoelectric thin film 3a is distorted due to acoustic
pressure or the like, polarization occurs inside thereof, and a voltage signal is extracted from the
wiring metal 5a connected to the electrode 4a and the wiring metal 5b connected to the
electrode 4b. Is possible. Similarly, when the piezoelectric thin film 3b is distorted, polarization
occurs inside thereof, and it becomes possible to extract voltage signals from the wiring metal 5a
connected to the electrode 4c and the wiring metal 5b connected to the electrode 4b.
[0008]
By the way, the residual stress of the piezoelectric thin film is released by the cantilever
structure, and as a result, the piezoelectric thin film warps, and the gap between the adjacent
piezoelectric thin films (gap between beams G) or the piezoelectric thin film (beam) side surface
and supporting substrate The size of the substantial gap is enlarged. When the piezoelectric
element is used as a microphone, the generation of a gap larger than such a design value lowers
the acoustic resistance and causes characteristic deterioration such as low frequency sensitivity
deterioration.
[0009]
Therefore, in order to solve this problem, instead of making the shape of the piezoelectric thin
film rectangular, it is made triangular, for example, by arranging each vertex of four triangles to
be located at the center, even if the piezoelectric thin film is warped. There is disclosed a
technique in which the same warp occurs in the adjacent piezoelectric thin films, and as a result,
the dimension of the gap between the adjacent piezoelectric thin films is not largely changed
(Patent Document 2).
[0010]
Patent No. 5707323 gazette Special table 2014-515214 gazette
[0011]
04-05-2019
3
In order to prevent the characteristic deterioration due to the residual stress of the piezoelectric
thin film, in the conventional piezoelectric element, the shape of the piezoelectric thin film is
triangular, and by arranging so as to center the vertex of four triangles, the dimension of the gap
of the piezoelectric thin film Made it possible not to make major changes.
However, in order to match the resonance frequency of each triangular beam, it is necessary to
combine triangles of the same shape, and the outer shape of the piezoelectric MEMS element is
limited to square, and there is a problem that the freedom of design is lost.
An object of the present invention is to provide a piezoelectric element with high sensitivity and
improved signal-to-noise ratio, while suppressing the influence of residual stress of the
piezoelectric thin film and solving the problem of limitation of the outer shape.
[0012]
In order to achieve the above object, the invention according to claim 1 of the present invention
is a piezoelectric element comprising a piezoelectric thin film whose both ends are fixed to a
supporting substrate, and a pair of electrodes disposed with the piezoelectric thin film interposed
therebetween. A plurality of sets of the pair of electrodes disposed with a part of the first
piezoelectric thin film interposed therebetween, and having a laminated structure including at
least a first piezoelectric thin film and a second piezoelectric thin film; A piezoelectric element, a
second piezoelectric element, and a third piezoelectric element, and a plurality of sets of the pair
of electrodes disposed so as to sandwich a portion of the second piezoelectric thin film, at least a
fourth piezoelectric element An element, a fifth piezoelectric element and a sixth piezoelectric
element are formed, and the first piezoelectric element, the second piezoelectric element and the
third piezoelectric element are connected from one end side to the other end of the both ends
Side by side, and The fourth piezoelectric element, the fifth piezoelectric element and the sixth
piezoelectric element are arranged in order from one end side to the other end of the both ends,
and the first piezoelectric element and the fourth piezoelectric element Element, the piezoelectric
elements of the second piezoelectric element and the fifth piezoelectric element, or the
piezoelectric elements of the third piezoelectric element and the sixth piezoelectric element are
connected in parallel, and the piezoelectric elements connected in parallel are in series And the
first piezoelectric element and the fourth piezoelectric element are vertically stacked on top of
each other, and the second piezoelectric element and the fifth piezoelectric element are vertically
stacked on top of each other. And the third piezoelectric element and the sixth piezoelectric
element are stacked in a vertically symmetrical manner.
04-05-2019
4
[0013]
The invention according to claim 2 of the present application is the piezoelectric element
according to claim 1, wherein the first to sixth piezoelectric elements are the first piezoelectric
element, the second piezoelectric element, and the third piezoelectric element. It is connected in
series that the fourth piezoelectric element, the fifth piezoelectric element and the sixth
piezoelectric element are connected in series, and the two sets of piezoelectric elements
connected in series are connected in parallel. To feature
[0014]
The invention according to claim 3 of the present invention is the piezoelectric element
according to any one of claims 1 or 2, wherein the group of piezoelectric elements connected in
parallel is a front surface and a rear surface of the first piezoelectric thin film or the second
piezoelectric thin film. Alternatively, it is characterized in that it is connected in series by a
continuous extension from the electrode of the piezoelectric element disposed between the films.
[0015]
According to a fourth aspect of the present invention, in the piezoelectric element according to
any one of the first to third aspects, when the piezoelectric thin film is curvedly displaced by
vibration, at least the regions divided by the inflection point of the displacement. Any of the first
piezoelectric element and the fourth piezoelectric element, and the second piezoelectric element
and the fifth piezoelectric element, or the third piezoelectric element and the sixth piezoelectric
element, which are stacked and formed symmetrically in the vertical direction. It is characterized
by the fact that it is arranged.
[0016]
The invention according to claim 5 of the present application is the piezoelectric element
according to any one of claims 1 to 4, characterized in that the piezoelectric thin film is a film
that vibrates by acoustic pressure.
[0017]
In the piezoelectric element of the present invention, the residual stress and temperature
fluctuation of the overlapping piezoelectric thin film are arranged by arranging the piezoelectric
element formed in the first piezoelectric thin film and the piezoelectric element formed in the
second piezoelectric thin film so as to overlap vertically symmetrically. Mutually offset the
piezoelectric voltage generated due to the influence of the residual stress of the piezoelectric thin
film and reduce the piezoelectric voltage by the first piezoelectric thin film and the piezoelectric
04-05-2019
5
voltage by the second piezoelectric thin film It is possible to raise the level of the output signal by
superimposing.
[0018]
Further, according to the present invention, by providing the double-supported beam structure in
which both ends of the piezoelectric thin film are fixed to the support substrate, large
deformation of the piezoelectric thin film is suppressed, and the shape is not limited to square
either. It is possible to secure the
[0019]
Furthermore, according to the present invention, when the piezoelectric thin film is bent and
deformed by vibration, the piezoelectric element and the second piezoelectric thin film are
formed on the first piezoelectric thin film for each of the regions divided by the inflection point
of the displacement. By arranging the set with the piezoelectric element, the piezoelectric
element is separated in each of the divided areas in the tensile stress area and the compressive
stress area generated in the beam extension direction, and the voltage signal generated in each
area is superimposed It becomes possible to convert to electrical energy efficiently and to take
out by connecting as shown in FIG.
[0020]
According to the present invention, the connection between the piezoelectric elements can be
performed by extending the electrodes of the piezoelectric elements, and the connection between
the piezoelectric elements can be efficiently performed without requiring any connection means
such as through holes which affect the displacement of the piezoelectric thin film. It has the
advantage that it can be converted into energy.
[0021]
In particular, when the piezoelectric thin film of the piezoelectric element of the present
invention is set to a thickness that vibrates by acoustic pressure and used as an acoustic
transducer, improvement in signal-to-noise ratio with high sensitivity is expected.
[0022]
It is a top view of the piezoelectric element of a 1st example of the present invention.
04-05-2019
6
It is sectional drawing of the piezoelectric element of the 1st Example of this invention.
It is explanatory drawing in case an acoustic pressure signal is applied to a piezoelectric element,
and a piezoelectric thin film displaces.
It is a graph which shows the film thickness dependence of the piezoelectric thin film which
consists of aluminum nitride of signal noise ratio.
It is a graph which shows the relationship between a microphone characteristic and slit width.
It is a figure explaining the manufacturing method of the piezoelectric element of the 1st
Example of this invention.
It is a graph which shows the film thickness dependence of the piezoelectric thin film which
consists of scandium aluminum nitride of signal noise ratio.
It is sectional drawing of the piezoelectric element of the 2nd Example of this invention.
It is explanatory drawing of the conventional piezoelectric MEMS element.
[0023]
The piezoelectric element of the present invention has a double-supported beam structure in
which both ends of the piezoelectric thin film are fixed to a support substrate.
The piezoelectric thin film has a laminated structure including at least two piezoelectric thin
films.
In each piezoelectric thin film, a plurality of piezoelectric elements in which electrodes are
04-05-2019
7
disposed so as to sandwich a part thereof are formed, and each piezoelectric element is
connected in parallel or in series. In the present invention, in particular, the piezoelectric
elements are arranged so as to overlap in the vertical symmetry. According to the present
invention, with the above-described configuration, residual voltages and piezoelectric voltages
generated due to temperature fluctuations are mutually offset by the outputs of the vertically
overlapping piezoelectric elements, thereby improving the signal noise ratio. Furthermore, the
signal can be efficiently extracted by arranging the pair of piezoelectric elements overlapping
vertically symmetrically at a predetermined position. Hereinafter, the case where the
piezoelectric element of the present invention is configured as an acoustic transducer will be
described in detail as an example.
[0024]
FIG. 1 is a plan view of a piezoelectric element according to a first embodiment of the present
invention, and FIG. 2 is a cross-sectional view of the piezoelectric element shown in FIG. As
shown in FIG. 2, piezoelectric thin films 3a and 3b are laminated and formed on a silicon
substrate 1 as a support substrate via an insulating film 2 made of a silicon oxide film (SiO2). In
this embodiment, as shown in FIG. 1, slits 6 extending in the lateral direction of the drawing are
formed in order to provide a double-supported beam structure. For example, aluminum nitride
(AlN) can be used as the piezoelectric thin film, and the crystal orientation (piezoelectric
orientation) is formed in the same direction in each of the laminated piezoelectric thin films.
[0025]
The electrode 4a1 and the electrode 4a2 are formed on the back surface side of the piezoelectric
thin film 3a, and the electrode 4a1 is connected to the wiring electrode 5a. The electrode 4a2 is
not connected to the wiring electrode 5a or other electrodes, and is in a floating state. Further, an
electrode 4b1 and an electrode 4b2 are formed on the upper surface side of the piezoelectric
thin film 3a and on the lower surface side (corresponding to between films) of the piezoelectric
element 3b, and the electrode 4b2 is connected to the wiring electrode 5b. The electrode 4b1 is
not connected to the wiring electrode 5b or other electrodes, and is in a floating state. Further, an
electrode 4c1 and an electrode 4c2 are formed on the upper surface side of the piezoelectric thin
film 3b, the electrode 4c1 is connected to the wiring electrode 5a, and the electrode 4c2 is not
connected to the wiring electrode 5a or other electrodes. , Is in a floating state. The electrode can
be formed of a metal thin film such as molybdenum (Mo), platinum (Pt), titanium (Ti), iridium (Ir),
ruthenium (Ru) or the like.
04-05-2019
8
[0026]
According to this structure, the piezoelectric element C1 (corresponding to the first piezoelectric
element) is formed in a region where the electrode 4a1, the piezoelectric thin film 3a
(corresponding to the first piezoelectric thin film), and the electrode 4b1 overlap. Similarly,
piezoelectric element C2 (corresponding to a second piezoelectric element) in the area where
electrode 4a2, piezoelectric thin film 3a and electrode 4b1 overlap, and piezoelectric element C3
(third piezoelectric in the area where electrode 4a2, piezoelectric thin film 3a and electrode 4b2
overlap. Element corresponds to a piezoelectric element C4 (corresponding to a fourth
piezoelectric element), an electrode 4c2, a piezoelectric thin film 3b and an electrode 4b1 in a
region where the electrode 4c1, the piezoelectric thin film 3b (corresponding to a second
piezoelectric thin film) and the electrode 4b1 overlap. Is formed in a region where the
piezoelectric element C5 (corresponding to the fifth piezoelectric element), the electrode 4c2, the
piezoelectric thin film 3b, and the electrode 4b2 overlap.
[0027]
As a result, the first piezoelectric element C1 and the fourth piezoelectric element C4 are
connected in parallel, and the series connection of the second piezoelectric element C2 and the
third piezoelectric element C3 and the fifth piezoelectric element C5 and the sixth piezoelectric
The series connection of the elements C6 is connected in parallel, and the piezoelectric elements
connected in parallel are connected in series between the wiring electrode 5a and the wiring
metal 5b.
[0028]
Here, for example, the first piezoelectric element C1 and the second piezoelectric element C2
share the electrode 4b1 constituting the piezoelectric element, so that the electrode 4b1 does not
overlap the opposing electrodes (the electrodes 4a1 and 4a2 respectively). Connected by an area
(corresponding to an extension).
Similarly, the fourth piezoelectric element C4 and the fifth piezoelectric element C5 share the
electrode 4b1 constituting the piezoelectric element, so that the area of the electrode 4b1 which
does not overlap the opposing electrodes (the electrodes 4c1 and 4c2 respectively) Connected by
(equivalent to an extension). Further, the second piezoelectric element C2 and the third
piezoelectric element C3 are not overlapped with the electrodes facing each other by the
04-05-2019
9
electrode 4a2, and the fifth piezoelectric element C5 and the sixth piezoelectric element C6 are
not overlapped with the opposing electrodes by the electrode 4c2. They are connected by the
extension) or by the electrode 4c2 area (corresponding to the extension). With such a connection,
it is not necessary to form connection means for affecting the displacement of the piezoelectric
thin film, such as through holes, in the piezoelectric thin film.
[0029]
As is apparent from FIG. 2, the first piezoelectric element C1 and the fourth piezoelectric element
C4, the second piezoelectric element C2 and the fifth piezoelectric element C5, and the third
piezoelectric element C3 and the sixth piezoelectric element C6. Is vertically symmetrical with
respect to a plane passing through the center of the electrode 4b1 and the electrode 4b2 in the
thickness direction, at least in the region for forming each piezoelectric element.
[0030]
A part of the back surface side of the silicon substrate 1 is removed to form a void 7, and the
electrodes 4 a 1 and 4 a 2 and the piezoelectric thin film 3 a are exposed in the void 7.
The holes 7 communicate with the surface side of the silicon substrate 1 through the slits 6
shown in FIG.
[0031]
By configuring in this manner, the piezoelectric element of the present embodiment has a doublesupported beam structure in which a plurality of electrode pairs are formed on a piezoelectric
thin film of which both ends are supported on a silicon substrate 1 (supporting substrate).
[0032]
When the piezoelectric element of the present invention is configured as an acoustic transducer,
acoustic pressure is applied from the holes 7 formed in the silicon substrate 1.
The beam structure including the piezoelectric thin film subjected to the acoustic pressure is
curved and displaced upward. As a result, tensile stress and compressive stress are generated in
04-05-2019
10
the aluminum nitride constituting the piezoelectric thin film.
[0033]
FIG. 3 shows an example where an acoustic pressure signal is applied and the piezoelectric thin
film is displaced. In this case, two inflection points are generated and divided into three regions
depending on the direction of stress on the piezoelectric thin film. For example, in the regions 1
and 3, the curved displacement occurs in a downward convex shape, and a tensile stress is
generated in the first piezoelectric thin film 3a and a tensile stress is generated in the second
piezoelectric thin film 3b. On the other hand, in the region 2, the curved displacement occurs in
an upward convex shape, and a compressive stress is generated in the first piezoelectric thin film
3a, and a tensile stress is generated in the second piezoelectric thin film 3b.
[0034]
In the piezoelectric element of this embodiment, as shown in FIG. 2, the piezoelectric elements C1
and C4 are connected in series, and the piezoelectric elements C2 and C3 are connected in series
and the piezoelectric elements C5 and C6 are connected in series. Each is connected in parallel,
and has a vertically symmetrical structure. Therefore, the voltages generated in the respective
regions 1 to 3 are opposite in polarity and have the same value, so that the in-phase voltage
caused by the residual stress and the temperature fluctuation is canceled out.
[0035]
As a result, the output signal (voltage) of each region based on application of the acoustic
pressure signal is superimposed and added without including the signal caused by the residual
stress or the temperature fluctuation, and the output voltage (Vout) with respect to the acoustic
pressure (Pa) It is possible to increase the sensitivity as an acoustic transducer defined by the
ratio (Vout / Pa).
[0036]
It is desirable that the size and the like of each electrode be optimized from the viewpoint of
maximizing the signal noise ratio.
04-05-2019
11
This is the size of each electrode so as to maximize the energy (Cout · Vout <2> / 2) stored in the
equivalent capacitor when the capacitance of the equivalent capacitor viewed from the wiring
electrodes 5a and 5b is Cout. You can decide the
[0037]
Specifically, one design example of the dimensions in the case of a rectangular double-ended
beam, the film thickness of each piezoelectric thin film, and the size of an electrode will be
shown. For example, it is assumed that the input signal is human voice, and the resonance
frequency of the beam is 20 kHz. Moreover, it is set as the planar dimension supposing mounting
in electronic devices like a smart phone. The length of the double-supported beam
(corresponding to the length of the slit in FIG. 1) is 0.7 mm, and the width (upper and lower in
FIG. 1) is 1.4 mm. The thickness of each of the piezoelectric thin films 3a and 3b made of
aluminum nitride is 0.5 μm, and the thickness of each of the electrodes 4a1 to 4c1 and 4a2 to
4c2 made of molybdenum is 0.1 μm. The extension lengths from the supporting ends (ends of
the holes 7) of the electrodes 4a1, 4c1 and 4b2 are both 90 μm, and the lengths from the
supporting ends of the electrodes 4b1, 4a2 and 4c2 to the electrode ends are both 500 μm. The
width of the slit 6 is 1 μm.
[0038]
The thickness of the piezoelectric thin films 3a and 3b and the length of the both-end beam (slit)
can be determined as follows. FIG. 4 is a graph showing the film thickness dependency of the
piezoelectric thin film made of aluminum nitride, of the signal noise ratio. The resonance
frequency was constant (20 kHz), the beam width was 1.4 mm, and the slit width was 1 μm. In
addition, in consideration of being limited by the size of the mounting case when mounted on an
electronic device such as a smartphone, the volume of the hole is set to a relatively small value of
3 mm <2>. In addition to the piezoelectric element (acoustic transducer) of the present invention,
it is necessary to mount an amplification circuit for processing an output signal of the
piezoelectric element in a small case such as a smartphone. Therefore, in consideration of the
allowable chip size of the piezoelectric element, the following examination was performed for the
cases where the beam length is 0.6 mm, 0.7 mm, and 0.8 mm.
[0039]
04-05-2019
12
As shown in FIG. 4, when the film thickness of each piezoelectric thin film made of aluminum
nitride is made smaller than 0.4 μm, the signal noise ratio improves, though slightly. However,
since the substantial acousto-electrical conversion coefficient is limited due to the acoustic
compliance of the holes, it is understood that the improvement effect due to thinning is not
remarkable and the dependency on the length of the beam is not large. Conversely, if the
thickness of the piezoelectric thin film is increased, the signal-to-noise ratio drops sharply. Also,
the dependence on the length of the beam becomes remarkable. That is, it can be seen that, in
order to keep the resonance frequency constant, it is necessary to increase the length of the
beam when the thickness of the piezoelectric thin film is increased. In the example shown in FIG.
4, when the thickness of the piezoelectric thin film is 0.5 μm, the length of the beam is 0.7 mm,
and when the thickness of the piezoelectric thin film is 0.6 μm, the length of the beam is It turns
out that it is preferable to set it as 0.8 mm.
[0040]
Next, the slit width will be described. FIG. 5 is a graph showing the relationship between the rate
of decrease in sensitivity at 100 Hz and the slit width based on the sensitivity at 1 kHz, which is
one of the main characteristics of the microphone. Similarly to the above, the resonance
frequency of the double-supported beam is 20 kHz, the length of the beam is 0.7 mm, the width
is 1.4 mm, and the volume of the holes is 3 mm <2>. The relationship between the piezoelectric
thin film made of aluminum nitride and the slit width was examined for the cases of 0.4 μm, 0.5
μm, and 0.6 μm.
[0041]
As shown in FIG. 5, the dependency on the thickness of the piezoelectric thin film is hardly seen,
and when the slit width exceeds 1 μm, the sensitivity drop at 100 Hz becomes remarkable. The
desensitization becomes even more severe at frequencies below 100 Hz. That is, it is understood
that the slit width needs to be 1 μm or less in order to suppress the decrease in sensitivity at low
frequencies.
[0042]
As described above, in order to increase the sensitivity, the length of the double-supported beam,
the thickness of the piezoelectric thin film, the slit width, and the like may be appropriately
adjusted and set.
04-05-2019
13
[0043]
The piezoelectric element of the present invention can be formed using a conventional method of
manufacturing a semiconductor device.
FIG. 6 is an explanatory drawing of the method of manufacturing the piezoelectric element of this
embodiment. First, an insulating film 2 made of a silicon oxide film (SiO 2) is formed on a silicon
substrate 1 by a thermal oxidation method. On the insulating film 2, a molybdenum (Mo) film
having a thickness of 0.1 μm is deposited by sputtering, and patterning is performed by a
normal lithography method to form an electrode 4a1 and an electrode 4a2 (FIG. 6a).
[0044]
Thereafter, an aluminum nitride film having a thickness of 0.5 μm is stacked on the entire
surface by sputtering to form a piezoelectric thin film 3a corresponding to a first piezoelectric
thin film. Thereafter, a molybdenum (Mo) film having a thickness of 0.1 μm is stacked on the
piezoelectric thin film 3a by sputtering, and patterning is performed by a normal lithography
method to form an electrode 4b1 and an electrode 4b2. Further, an aluminum nitride film having
a thickness of 0.5 μm is stacked on the entire surface by sputtering to form a piezoelectric thin
film 3b corresponding to a second piezoelectric thin film. A molybdenum (Mo) film having a
thickness of 0.1 μm is laminated by sputtering, and patterning is performed by a normal
lithography method to form an electrode 4c1 and an electrode 4c2 (FIG. 6b).
[0045]
A portion of the piezoelectric thin films 3a and 3b is removed by etching to form a wiring
electrode 5a connected to the electrode 4a1 and the electrode 4c1 and a wiring electrode 5b
connected to the electrode 4b2. The wiring electrodes 5a and 5b are made of aluminum and can
be formed by a normal lithographic method (FIG. 6c). When the piezoelectric thin films 3a and
3b are partially etched away, the piezoelectric thin films 3a and 3b in the portion corresponding
to the slit 6 shown in FIG. 1 are also etched away to form a recess, and the insulating film 2 is
formed on the bottom. Leave it exposed.
04-05-2019
14
[0046]
Finally, a part of the back surface of the silicon substrate 1 is removed by a dry etching method,
and a part of the exposed insulating film 2 is also removed to form a void 7. In the holes 7, the
electrode 4a1, the electrode 4b1 and a part of the piezoelectric thin film 3a previously formed
are exposed. By the formation of the holes 7, the insulating film 2 exposed at the bottom of the
recess formed to form the slit 6 is also removed, and the slit 6 in which the front and back sides
of the piezoelectric thin film penetrate is formed. By this slit 6, the piezoelectric thin film has a
double-supported beam structure (FIG. 6d).
[0047]
As mentioned above, although the piezoelectric element of this example and its manufacturing
method were explained, it is needless to say that the present invention is not limited to aluminum
nitride as a piezoelectric thin film. Table 1 shows Young's moduli that affect the characteristics of
a piezoelectric microphone for representative piezoelectric materials such as aluminum nitride,
scandium aluminum nitride (Al1-xScxN), zinc oxide (ZnO), and lead zirconate titanate (PZT). It is
the table | surface which compared material constants, such as a transverse piezoelectric
distortion coefficient.
[0048]
[0049]
The figure of merit (FOM) corresponding to the signal-to-noise ratio shown in Table 1 is
represented by the ratio of the coupling coefficient (k31 <2>) to the loss angle (tan δ), and the
larger the value is, the more nearly it is proportional to the value Can be expected to improve the
signal noise ratio.
As shown in Table 1, it is understood that aluminum nitride has a 6 to 40-fold higher
performance index than zinc oxide and lead zirconate titanate, and is a material suitable for a
piezoelectric transducer. In addition, scandium aluminum nitride (Al1-xScxN) in which scandium
is added to aluminum nitride is known to improve the horizontal piezoelectric strain coefficient
04-05-2019
15
more than aluminum nitride. For example, when the ratio of scandium is 35%, the figure of merit
is Is expected to be improved about seven times more than aluminum nitride.
[0050]
FIG. 7 is a graph showing the film thickness dependency of a piezoelectric thin film made of
scandium aluminum nitride (Al 1 -xSc x N: x = 0.35) with a signal-to-noise ratio. As compared
with the graph showing the film thickness dependency of the piezoelectric thin film made of
aluminum nitride shown in FIG. 4, it can be seen that an improvement of the signal noise ratio of
about 8 dB corresponding to seven times the figure of merit can be expected. It was also
confirmed that the film thickness dependency showed the same tendency regardless of the type
of piezoelectric thin film. Specifically, when the thickness of the piezoelectric thin film is 0.4 μm,
the length of the beam is 0.6 mm, and when the thickness of the piezoelectric thin film is 0.6
μm, the length of the beam is 0. When the thickness of the piezoelectric thin film is 7 mm and
the thickness of the piezoelectric thin film is 0.7 μm, it is understood that the length of the beam
is preferably 0.8 mm.
[0051]
Next, a second embodiment of the present invention will be described. FIG. 8 is a cross-sectional
view of a piezoelectric element according to a second embodiment of the present invention. This
embodiment differs from the previously described piezoelectric element shown in FIG. 2 in that
the dielectric film 8, the electrode 4d1, the electrode 4d2 and the like are provided.
[0052]
That is, as shown in FIG. 8, the piezoelectric thin film 3a, the dielectric film 8 and the
piezoelectric thin film 3b are laminated and formed on the silicon substrate 1 as a support
substrate via the insulating film 2 made of silicon oxide film (SiO2). ing. In this embodiment, in
order to form a double-supported beam structure, slits 6 extending in the lateral direction of the
drawing are formed as described in FIG. For example, aluminum nitride (AlN) can be used as the
piezoelectric thin film, and the crystal orientation (piezoelectric orientation) is formed in the
same direction.
04-05-2019
16
[0053]
In the piezoelectric element of this embodiment, an electrode 4a1 and an electrode 4a2 are
formed on the back surface side of the piezoelectric thin film 3a, and the electrode 4a1 is
connected to the wiring electrode 5a. The electrode 4a2 is not connected to the wiring electrode
5a or other electrodes, and is in a floating state. Further, an electrode 4b1 and an electrode 4b2
are formed on the upper surface side of the piezoelectric thin film 3a, and the electrode 4b2 is
connected to the wiring electrode 5b. The electrode 4b1 is not connected to the wiring electrode
5b or other electrodes, and is in a floating state.
[0054]
A dielectric film 8 is stacked on the piezoelectric thin film 3a and the electrodes 4b1 and 4b2,
and an electrode 4d1 and an electrode 4d2 are formed on the dielectric film 8. The electrodes
4d1 and 4d2 have the same shape as the electrodes 4b1 and 4b2 formed earlier. The electrode
4d2 is connected to the wiring electrode 5b, and the electrode 4d1 is not connected to the wiring
electrode 5b or other electrodes, and is in the same floating state.
[0055]
Specifically, an electrode 4d1 and an electrode 4d2 are formed on the upper surface side of the
dielectric film 8 and on the lower surface side of the piezoelectric element 3b (corresponding to
between films), and the electrode 4d2 is connected to the wiring electrode 5b. ing. The electrode
4d1 is not connected to the wiring electrode 5b or other electrodes, and is in a floating state.
Further, an electrode 4c1 and an electrode 4c2 are formed on the upper surface side of the
piezoelectric thin film 3b, the electrode 4c1 is connected to the wiring electrode 5a, and the
electrode 4c2 is not connected to the wiring electrode 5a or other electrodes. , Is in a floating
state. The electrode can be formed of a metal thin film such as molybdenum (Mo), platinum (Pt),
titanium (Ti), iridium (Ir), ruthenium (Ru) or the like.
[0056]
According to this structure, the piezoelectric element C1 (corresponding to the first piezoelectric
element) is formed in a region where the electrode 4a1, the piezoelectric thin film 3a
04-05-2019
17
(corresponding to the first piezoelectric thin film), and the electrode 4b1 overlap. Similarly, the
piezoelectric element C2 (corresponding to the second piezoelectric element) is in the area where
the electrode 4a2, the piezoelectric thin film 3a and the electrode 4b2 overlap, and the
piezoelectric element C3 (third area) in the area where the electrode 4a2, the piezoelectric thin
film 3a and the electrode 4b2 overlap. Of the piezoelectric element C4 (corresponding to the
fourth piezoelectric element) in the region where the electrode 4c1, the piezoelectric thin film 3b
(corresponding to the second piezoelectric thin film) and the electrode 4d1 overlap, the electrode
4c2, the piezoelectric thin film 3b The piezoelectric element C5 (corresponding to the fifth
piezoelectric element) is formed in the area where the electrodes 4d1 overlap, and the
piezoelectric element C6 (corresponding to the sixth piezoelectric element) is formed in the area
where the electrode 4c2, the piezoelectric thin film 3b, and the electrode 4d2 overlap. Be done.
[0057]
As a result, the first piezoelectric element C1, the second piezoelectric element C2 and the third
piezoelectric element C3 are connected in series between the wiring electrode 5a and the wiring
metal 5b. Similarly, the fourth piezoelectric element C4, the fifth piezoelectric element C5 and the
sixth piezoelectric element C6 are connected in series between the wiring electrode 5a and the
wiring metal 5b. Furthermore, a set of these piezoelectric elements connected in series is also
connected in parallel.
[0058]
Even if formed in this manner, as is apparent from FIG. 8, the first piezoelectric element C1 and
the fourth piezoelectric element C4, the second piezoelectric element C2 and the fifth
piezoelectric element C5, and the third piezoelectric element C3. The sixth piezoelectric element
C6 has a vertically symmetrical structure with respect to a plane passing through the center of
the dielectric film 8 in the thickness direction, at least in the region where each piezoelectric
element is formed.
[0059]
In the back surface side of the silicon substrate 1, a void 7 is formed by removing a part of the
hole, and the electrodes 4a1 and 4a2 and the piezoelectric thin film 3a are exposed in the void 7.
04-05-2019
18
The holes 7 communicate with the surface side of the silicon substrate 1 through the slits 6
shown in FIG.
[0060]
With this configuration, even in the piezoelectric element of this embodiment, a doublesupported beam structure in which a plurality of electrode pairs are formed on a piezoelectric
thin film of which both ends are supported on a silicon substrate 1 (supporting substrate) can be
obtained.
[0061]
When the piezoelectric element of the present embodiment is configured as an acoustic
transducer, acoustic pressure is applied from the holes 7 formed in the silicon substrate 1.
The beam structure including the piezoelectric thin film subjected to the acoustic pressure is
curved and displaced upward. As a result, tensile stress and compressive stress are generated in
the aluminum nitride constituting the piezoelectric thin film.
[0062]
However, as in the first embodiment, the piezoelectric elements C1 and C4, the piezoelectric
elements C2 and C5, and the piezoelectric elements C3 and C6 have vertically symmetrical
structures, respectively, as shown in FIG. As in the case described above, since the voltages
generated in the respective regions have opposite polarities and the same value, it is possible to
cancel the in-phase voltage caused by the residual stress or the temperature fluctuation.
[0063]
As a result, the output signal (voltage) of each region based on application of the acoustic
pressure signal is superimposed and added without including the signal caused by the residual
stress or the temperature fluctuation, and the output voltage (Vout) with respect to the acoustic
pressure (Pa) It is possible to increase the sensitivity as an acoustic transducer defined by the
ratio (Vout / Pa).
04-05-2019
19
In particular, since the piezoelectric signal is extracted from the uppermost piezoelectric thin film
3b and the lowermost piezoelectric thin film 3a of the laminated structure and the piezoelectric
signal is extracted from a portion where the stress is relatively large, it is expected that the signal
noise ratio is further improved. Be done.
[0064]
It is desirable that the size and the like of each electrode be optimized from the viewpoint of
maximizing the signal noise ratio. This is the size of each electrode so as to maximize the energy
(Cout · Vout <2> / 2) stored in the equivalent capacitor when the capacitance of the equivalent
capacitor viewed from the wiring electrodes 5a and 5b is Cout. The same goes for what you need
to decide. Further, the thickness and material of the dielectric film may be appropriately selected
in order to obtain desired characteristics. The dielectric film may be aluminum nitride. When
three layers of aluminum nitride are stacked, the thickness of each layer may be 0.33 μm.
[0065]
When the inflection point of the displacement is 2 or more when the bending displacement
occurs due to the vibration of the piezoelectric thin film, the present invention is not limited to
the above embodiment, and the number of piezoelectric elements is increased for each area
divided by the inflection point. Alternatively, a plurality of elements may be arranged in each
area.
[0066]
By the way, when using a piezoelectric element as in the present invention as an acoustic
transducer, an amplifier circuit for processing a signal output from the piezoelectric element is
required.
In general, integrated circuits are formed on different silicon substrates and mounted on the
mounting substrate as separate elements. In the piezoelectric element of the present invention,
there is no problem in forming an element for signal processing, for example, on the support
substrate. In that case, the piezoelectric thin film may be used as an interlayer insulating film,
and the electrode may be used as a wiring metal.
04-05-2019
20
[0067]
1: silicon substrate, 2: insulating film, 3a, 3b: piezoelectric thin film, 4a, 4b, 4c, 4d: electrode, 5a,
5b: wiring electrode, 6: slit, 7: void, 8: dielectric film
04-05-2019
21
Документ
Категория
Без категории
Просмотров
0
Размер файла
34 Кб
Теги
jp2018026445
1/--страниц
Пожаловаться на содержимое документа